Platelet accumulation at sites of vascular injury is a dynamic process that mediates formation of both the primary hemostatic plug and pathologic thrombus formation. The mechanisms by which platelet surface proteins direct platelet recruitment to thrombi under flow conditions have been studied in detail (Ruggeri et al., 1999, Blood 94, 172–8; Savage et al., 1998, Cell 94, 657–66). In addition to directing initial platelet adhesion, cell-surface receptor interactions activate intracellular signaling (Yap et al., 2002, Blood 99, 151–8). Intracellular signaling stimulates the release of thrombogenic substances from platelet granules. Signaling also mediates activation of the platelet integrin a11bβ3 that facilitates firm adhesion of platelets to thrombi (Nesbitt et al., 2002, J Biol Chem 277, 2965–72; Schoenwaelder et al., 2002, J Biol Chem 5, 5).
Arterial thrombosis mediates tissue infarction in coronary artery disease, cerebrovascular disease, and peripheral vascular disease, and, thus, is the single most common cause of morbidity and mortality in the United States. Platelets are key mediators of arterial thrombosis. Thus, the identification of compounds that inhibit platelet function is of great importance to medicine.
Platelets form the body's primary means of hemostasis and, as such, have developed an elaborate mechanism of surveying the vasculature for defects in endothelial integrity. This mechanism involves the ability to respond to subendothelial matrices (Savage et al., 1998, Cell 94, 657–666), shear forces (Fredrickson et al., 1998, Blood 92, 3684–3693), neighboring platelets (Shattil et al., 1985, Blood 66, 92–98), the adrenal axis (Larsson et al., 1992, Thromb Haemost 68, 687–693), as well as soluble proteinacious, nucleotide, and lipid signals. Despite this plethora of physiologic activators, the platelet has only a small repertoire of major functional outputs. Upon activation, platelets change shape, aggregate, and secrete their granular contents. The process of platelet activation involves the expression of activities not shared by functionally merit resting platelets, including, for example, ATP release, serotonin release, lysosomal release, alpha granule release, dense granule release, and cell surface expression of markers of activated platelets (including, but not limited to CD9, GPIb, GPIIb, GPIIIa, CDIa-IIa, P-selectin, PECAM-1, activated GPIIb/IIIa, and vitronectin receptor). In addition, platelet activation results in the aggregation of platelets with each other and with non-platelet surrounding cells. The granular contents of platelets supply additional adhesion molecules, growth factors, coagulation enzymes and other specialized molecules instrumental in the process of thrombus formation and the initiation of the healing process.
In addition to coronary artery disease/myocardial infarction, cerebrovascular disease and peripheral vascular disease, diseases and disorders associated with inappropriate platelet activity and arterial thrombosis also include, for example, stable and unstable angina, transient ischemic attacks, placental insufficiency, unwanted thromboses subsequent to surgical procedures (e.g., aortocoronary bypass surgery, angioplasty and stent placement, and heart valve replacement), or thromboses subsequent to atrial fibrillation. Inhibitors of platelet activity can provide therapeutic and preventive benefits for each of these diseases or disorders. It is also possible that inappropriate platelet activation plays a role in venous thrombosis, such that platelet inhibitors can be useful for the treatment or prevention of disorders associated with such thromboses.
A connection is emerging between platelet activation and inflammation, particularly allergic inflammation (e.g., in asthma) and inflammation at the sites of atherosclerotic damage. See, for example: Rinder & Fitch, 1996, J Cardiovasc Pharmacol 27, Suppl. 1:S6–12 (investigating the role of complement components in activation of platelet and polymorphonuclear neutrophils by cardiopulmonary bypass); Palabrica et al., 1992, Nature 359, 848–851 (P-selectin mediates leukocyte adhesion to platelets in vivo, and the bound leukocytes promote fibrin deposition); Papayianni et al., 1995, Kidney Int 47, 1295–1302 (reduction of platelets reduces generation of immune modulator lipoxin A4 generation during experimental immune complex-mediated glomerulonephritis); Bazzoni et al., 1991, Haematologica 76, 491–499 (review describing the elaborate cross-talk between platelets and neutrophils in thrombotic and inflammatory diseases); and Kazura, 1989, J Lab Clin Med 114, 469–470 (editorial on the platelet-neutrophil interaction and modulation of the inflammatory response). Therefore, compounds that inhibit platelet activation can also be useful in the treatment or prevention of disorders involving inflammation.
There are a number of agents presently available that target platelet function. For example, aspirin is a relatively weak platelet inhibitor. However, aspirin can cause life-threatening allergic reactions in sensitive individuals.
Another platelet inhibiting agent is ticlopidine (Ticlid™, Roche Pharmaceuticals). Because it requires the production of active metabolites to be effective, the effect of ticlopidine is delayed 24–48 hours. The drug can also cause thrombotic thrombocytopenic purpura, a life-threatening condition, as well as nausea, abdominal pain, dyspepsia, diarrhea and skin rash.
Clodiprogel (Plavix™, Bristol-Meyers Squibb/Sanofi Pharmaceuticals) is another platelet inhibitor that requires the generation of active metabolites for its therapeutic efficacy. Therefore, clodiprogel also has a delay of 24–48 hours for its effect. Clodiprogel can also cause thrombotic thrombocytopenia purpura as well as agranulocytopenia, both life-threatening conditions. The drug has also been associated with rash, edema, hypertension, hypercholesterolemia, nausea, abdominal pain, dyspepsia, diarrhea, urinary tract infections, liver enzyme elevations and arthralgia.
The platelet inhibitory agents Abiximab and c7E3 Fab (Reopro abciximab™, manufacturer—Centocor B. V., distributor—Eli Lilly and Co.) are only available in a parenteral form. The drugs can cause severe thrombocytopenia. Both have a very long half-life and, therefore, complicate surgery that is sometimes required in the setting of life-threatening arterial occlusion (e.g., emergent cardiac surgery in the setting of a myocardial infarction).
Tirofiban (Aggrastat™, Merck and Co., Inc.) is another platelet inhibitory agent that is only available in a parenteral form. Tirofiban can cause thrombocytopenia, coronary artery dissection, bradycardia and edema, as well as dizziness and vasovagal reactions.
Eptifibatide (Integrilin™, COR Therapeutics, Inc., Key Pharmaceuticals Inc.) is another platelet inhibitory agent that is only available for parenteral administration. It can cause hypotension.
There is only limited clinical experience with the oral anti-GPIIbIIIa agents lamifiban, sibrafiban, orofiban and xemilofiban. Similarly, clinical experience is limited with the phosphodiesterase inhibitors cilostazol, trapidil and trifusal. There is more clinical experience with the phosphodiesterase inhibitor dipyridamole, but its activity is so weak that it is not frequently used.
There is a need in the art for additional platelet inhibitory agents for the treatment and prevention of diseases or disorders characterized by platelet activation and thrombosis.